At the Musée de la Musique in Paris, a major exhibition dedicated to the late Serge Gainsbourg has just opened. The ne plus ultra of Gallic style and insouciance, Gainsbourg is a national icon and the exhibition is a fitting tribute to his status. The only problem is, it's a Monday and the Musée de la Musique is closed. Step up, Jane Birkin, honorary Frenchwoman, former lover and muse to Gainsbourg, and wielder of considerable influence in the world of Parisian museum opening times.

“I'm sure they could open it up for you!” she says as she calls up the museum's director. Sure enough, they do. Such is the respect accorded to this English actress and singer in her adopted country - and the level to which Birkin and Gainsbourg are still associated.

“Every day, five times a day, I have taxi drivers talking to me about Serge,” says Birkin, who, with her short, tousled hair and expressive face free of make-up, looks not so different from the gamine that France first fell in love with back in 1969. “But I'm glad because when we were together I never felt he got his due. It was only when we were no longer together that the young people started to discover him. Now French culture is talked of as before and after Serge.”

One gets the impression that there's a before and after Serge with Birkin too. We are in the Japanese-themed bar of an hotel in Saint Germain, a neighbourhood with a blend of bourgeois chic and bohemian charm in which you feel that Birkin, whose accent reveals British officer-class roots despite 40 years in Paris, is very much at home. She is here to talk about Enfants d'hiver, her first album of entirely self-penned material. It uses elegant, understated settings to carry some very personal, intimate lyrics about childhood and growing older, and she's aware of the need to do justice to Gainsbourg's influence.

“I knew from Serge how much artistry it takes to put the maximum emotion into the minimum of words,” says Birkin on the challenge of writing songs after so many decades of singing other people's. “And I was thinking: maybe I should write my own stuff. Maybe if people knew that I was so terribly ordinary underneath my public image, they might feel the words had been written just for them.”

Perhaps remarkably, given that she's 62, Birkin believes that Enfants d'hiver is the first work she's done that isn't trying to please somebody else. “It's about the wildness of family holidays when I was a child, the wildness I've tried to give my own children, and the importance of family. To this day I couldn't imagine not being with my brother and sister and children for Christmas. I'm also not worrying if I still sing in an English accent after being in France for the past 40 years.”

“Do you still have an accent?” I ask.

“Oh God! It's atrocious. Unlike actresses such as Charlotte Rampling and Kristin Scott Thomas, I never learnt French grammar. Getting the word right was enough, and whether it had a ‘le' or a ‘la' before it didn't bother me at all.”

Most of the themes of Enfants d'hiver developed from Boxes (2007), Birkin's debut as a feature-film director. The story of a woman sifting through the memories of her life as she unpacks after moving home, it is clearly autobiographical.

“I was thinking about the nostalgia of childhood, of the majestic figures of your parents in the doorways of your life,” she says. She talks breathlessly as one memory after another tumbles out. “I remember being frightened at a place called Hop Castle,” she says. “Ma drove us in a Triumph Herald, and there were gloomy flowers like foxgloves around the place, and I gasped because we visited a tiny house made of skulls. The whole wonder of it is so exciting. We were winter children. It was the happiest time in my life.”

The personal spills over into the public in Birkin's life. It's hard to avoid, given that two of her daughters, Charlotte Gainsbourg and Lou Doillon, are successful actresses, and that the three men she's had daughters with - the English composer John Barry (father to her first, Kate), Gainsbourg and the film director Jacques Doillon - are all famous too.

Her activism is a product of personal tragedy. In 2001 her brother, the screenwriter and director Andrew Birkin, lost his 20-year-old son Anno in a car crash in Italy. Andrew set up an African charity in his son's honour and this led Jane to reassess her own responsibility as a public figure. A track about the Burmese political prisoner Aung San Suu Kyi on Enfants d'hiver, as well as a high-profile campaign for her release and a plea for governments to pull out investments from Burma, are among the products of this.

“I met Aung San Suu Kyi in Rangoon,” she says. “She is an extraordinarily funny and brilliant lady, and when I got back I went to see President Sarkozy to try to get him to pull [French oil giant] Total out of Burma. It didn't work. So I thought: perhaps the only force left is people. It comes down to the individual, what they can do.”

Birkin was one of the most powerful sex symbols of post-1968 Paris. Now she is a grandmother. One song, Madame, was written after strangers started calling her that respectful name for an older woman in France. She writes of the realisation: “It broke my heart/ Like a slap in the face.”

“It's about loneliness, really, and this idea of thinking you are still an adolescent because you haven't been aware of the outside of yourself changing. When someone is respectful enough to call you madame it's very kind, but it came as a shock. I'm no longer the pin-up Serge was forever taking pictures of.”

Yet you do feel that there's something of the eternal teenager in Jane Birkin. She was barely out of adolescence, after all, when she fell under the wing of Gainsbourg, and he continues to wield a strong influence on her. Before I leave, conversation inevitably returns to her former lover.

“Not long before he died there was a television special in which all these young children dressed like Serge,” she remembers with a wide, close-to-goofy smile. “They had their little cigarettes and beards and sang: ‘We just came to say we love you.' He started crying because he was someone that didn't love himself, who might have easily died without the knowledge that people in France did love him. Despite everything that I do, I always feel that he's never very far away.”

Enfants d'hiver by Jane Birkin is out on EMI today

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